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Stellar Wind
The stellar wind (or solar wind for the Sun) is a stream of matter ejecting a star. It is composed of charged ions, electrons and sometimes neutral atoms. The stellar wind is released with a speed from the star and then it is accelerated with the help of photons. Each star has a different type of stellar wind, influencing weather in its planetary system, terraforming possibilities of surrounding planets and space travel. Overall Stellar winds have a different composition. Around O - type stars and Wolf - Rayet Stars, the winds have speeds exceeding 1000 km/s, while around Red giants, their speed is low, around 10 km/s. Usually, they are made of hydrogen, which is the dominant element in most stars, but in case of Wolf - Rayet stars, they are often made of helium, carbon, oxygen or even silicon. Through stellar winds, stars lose matter. Red giants lose about one solar mass in a few thousands of years, while O - type and B - type stars lose one solar mass in tens to hundreds of millions of years. The highest mass loss is found to be in Wolf - Rayet stars. Even if on a stellar lifetime the amount of lass lost is significant, usually this is not enough to prevent them from their deaths. For example, a star with 10 times the mass of our Sun, will lose in its lifetime 2 to 3 solar masses. One of the only exceptions are low Wolf - Rayet stars. Stellar winds are not homogenous. Coronal mass ejections are stellar eruptions in which a stellar wind can increase its matter and energy output even millions of times. This is seen mostly in case of flares that occur in M - type stars and in case of huge mass losses that accompany Red giants. Even without flares and solar storms, the intensity of a stellar wind is not constant. Around stars like our Sun or smaller, the magnetosphere can keep plasma from solar winds captive for a limited time. Stellar winds move away from the star that formed them, until they are overwhelmed by the interstellar environment. There, they are decelerated and in the end stopped. Depending on the strength of a solar wind and on the strength of interstellar medium, they form a larger or smaller transit zone, with or without a bow shock area. The region surrounding a star where stellar winds are dominant and push the interstellar medium away, is known as heliosphere. Recent data from Voyager shows that Sun's heliosphere is really a sphere. Around brighter stars, there is not quite a sphere, but an elongated space, like the coma of a comet. Violent events, like a nova or a supernova, are also considered stellar winds. Stellar remnants, like White dwarfs, Neutron stars or Black holes are known not to have stellar winds. However, given their big mass and strong magnetic fields, they can keep huge radiation belts surrounding them, mainly if they have another stellar companion. And even if they are alone, their magnetic fields can keep the interstellar wind away, in the same way the Earth distorts the Solar wind. Stellar winds by star Wolf - Rayet Stars have the stronger winds of all. These stars are literally tearing apart, losing one solar mass in hundreds or thousands years. The fact that WR stars are very hot (30000 to 200000 K) also means that their strong radiation pressure accelerates their wind to speeds above 1000 km/s. Such strong winds have the power to erode atmospheres of their planets. It is possible that even Gas Giants are unable to hold their gaseous envelopes in place without a strong magnetic field. Red giants lose one solar mass in a few thousand years, but their winds are extremely slow (about 10 km/s). In such conditions, gas giants tend to accumulate matter from the solar winds, while an Earth - like planet would have its atmosphere slowly eroded. O - type stars lose about one solar mass in a few hundred million years, but their winds are very fast, reaching over 1000 km/s. These winds, combined with their powerful UV and X radiation, can erode atmospheres of smaller planets. B - type stars have smaller winds, losing one solar mass in a few billion years. Their winds are fast (about 1000 km/s) and pose a thread for any Habitable Zone Planet. Still, these winds are far smaller then those surrounding O - type stars. A - type stars have winds that are about 1000 times more powerful then those of our Sun. This is small enough for a Habitable Zone Planet without a magnetic field to hold its atmosphere long enough for millennia. F - type stars have winds less powerful then A - type stars, but still more powerful then those of our Sun. G - type stars are similar to our Sun. They have moderate stellar winds. It is estimated that if our Sun will never change its luminosity, it will lose all its mass in 10E+15 years, which is 10000 times longer then the Sun's lifetime. K - type stars have some of the slowest stellar winds known to date. As a result, their heliosphere is also far smaller. M - type stars are very controversial. Some scientists argue that their stellar winds are at least twice as strong as our Sun's, giving a maximum of 10 to 100 times stronger. If this is the case, giving their long lifetime, these stars might lose enough matter, up to the point where they no longer can sustain hydrogen fusion in their cores. On the other hand, it looks like heliospheres around some M - type stars are very small, extending up to only 10 AU, which can be explained only by very small and least powerful winds. What is known for sure is that these stars are flare stars. During flares, their brightness can increase even 100 times for a short period of time (about a few minutes). Flares occur with coronal mass ejections that can feed the stellar winds. So, maybe the intensity of the wind is strongly correlated with the frequency of flares. Brown Dwarfs are sometimes theorized to have small stellar winds. White dwarfs and Neutron stars don't have their own stellar winds, but giving their fast rotation period, they have magnetic fields. Given their strong gravity, they can accrete matter from the wind of a nearby star and they also can have radiation belts surrounding them. Formulas First of all, one must acknowledge that the intensity of a stellar wind will never be that of a theoretical model, because of many parameters: star rotation speed, convection currents, a stellar companion and metalicity (amount of heavier elements then hydrogen and helium). Still, there are two general formulas that can give a terraformer a general view of what to expect near a star: For red giants and WR stars: Log(-M) = -8.16+1.77Log(L/Ls)-1.68Log(Teff/K) where M''' is the amount of mass loss (in solar masses), '''L is star's luminosity, Ls is the luminosity of our Sun and Teff is the surface temperature of the star, in degrees K. For main sequence stars, the formula is: LogM = -12.76+1.3*Log(L/Ls) where M''' is the amount of mass loss, '''L is star's luminosity and Ls is our Sun's luminosity. Category:Habitable Factors Category:Math